This invention relates to refractory composites useful at high temperatures under corrosive environments and more particularly to high chromia-content refractories whose resistance to thermal shock-induced damage is improved by minor amounts of an additive such as ZrO.sub.2 having a temperature dependent phase change.
One use of refractory composites has been as refractory linings for the main pressure vessels of coal gasification systems operated to form synthetic natural gas from coal. In some systems and particularly slagging coal gasifiers, the linings are subject to corrosive environments from molten slag together with thermal shock associated with temperature fluctuations which may vary between 1400.degree.-1750.degree. C. Under these conditions, refractory failure may occur by corrosion, cracking, and spalling thereby reducing the service life of the lining.
High-chromia refractories have been developed with good resistance to corrosion from molten slag and therefore have advantages for use in slagging coal gasifiers. These refractories may be characterized by a Cr.sub.2 O.sub.3 content of greater than 50 wt. % with the Cr.sub.2 O.sub.3 being incorporated and/or mixed with other metal oxides such as ZnO, MgO, Al.sub.2 O.sub.3, SiO.sub.2, Na.sub.2 O, CaO, and the like. The particle size is in the order of 4-6 microns and above.
In the prior art, chromia compositions have also included various amounts of ZrO.sub.2 added for various purposes. In general, the ZrO.sub.2 has been in the stabilized form which has a limited dimensional change with temperature. Also, the presence of certain metal oxides such as CaO, MgO, Na.sub.2 O and the like in the refractory compositions tend to keep ZrO.sub.2 in stabilized form. As one illustration, U.S. Pat. No. 2,231,944 discloses a composition of 40-60 mole % CaO, 20-30 mole % Cr.sub.2 O.sub.3 and 20-30 mole % ZrO.sub.2 to impart resistance to hydration to the refractory. Another U.S. Pat. No. 3,837,870 discloses a composition of 1-74 wt. % Cr.sub.2 O.sub.3, 15-40 wt. % stabilized ZrO.sub.2, 3-76 wt. % Al.sub.2 O.sub.3 and 7.5-20 wt. % SiO.sub.2 for use in glass furnaces. U.S. Pat. No. 4,374,897 discloses a composition with 20-99 wt. % Cu.sub.2 O.sub.3 and various amounts of ZrSiO.sub.4, ZrO.sub.2 or SiO.sub.2 made in a carbon reduction atmosphere to provide a high density chromium oxide-based sintered body. From the description regarding ZrSiO.sub.4 and SiO.sub.2, the additives have low coefficients of thermal expansion with good heat shock resistance to compensate for some of the limitations of Cr.sub.2 O.sub.3. Russian Pat. No. 554,252 also discloses a composition of 75-92 wt. % magnesite-chromite clinker, 3-5 wt. % binder and 5-20 wt. % acicular ZrO.sub.2. Other patents related to the addition of ZrO.sub.2 to Cr.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 to ZrO.sub.2 are U.K. Pat. No. 1,118,073; U.S. Pat. Nos. 3,192,058; 3,281,137; 3,293,053; 3,309,209; 3,475,352; 4,141,743; and 4,492,766.
As reported in the above identified prior art, high chromia refractories provide good corrosion resistance to molten slag; however, they have limited resistance to thermal shock.
Another factor of importance for refractory composite compositions is that the particle size of the inclusions be small and have a narrow and uniform distribution. With the larger and nonuniform particle size distribution of the inclusions, the performance may become adversely effected.
Accordingly, one object of the invention is a high chromia refractory with improved resistance to thermal shock. A second object is a high chromia refractory with a combination of good corrosion resistance and thermal shock resistance.